Many communications systems currently use encryption to enhance security of the systems. As will be appreciated by those skilled in the art, these communication systems can be described according to the Open Systems Interconnection (OSI) model which includes seven layers including an application, presentation, session, transport, network, link, and physical layer. The OSI model was developed by the International Organization for Standardization (ISO) and is described in "The Basics Book of OSI and Network Management" by Motorola Codex from Addison-Wesley Publishing Company, Inc., 1993 (First Printing September 1992).
Communication systems include, but are not restricted, to cellular radio telephone communication systems, personal communication systems, paging systems, as well as wireline and wireless data networks. By way of example a cellular communication system will be described below; however, it will be appreciated by those skilled in the art that the encryption techniques described can be readily extended to other communication systems without departing from the scope and spirit of the present invention.
Turning now to cellular communication systems, these systems typically include subscriber units (such as mobile or portable units) which communicate with a fixed network communication unit (i.e., a base site) via radio frequency (RF) communication links. In cellular communication systems, the RF communication link is the primary target for cryptographic systems, because it is the most vulnerable to unauthorized introduction (spoofing) or extraction (eavesdropping) of information. It is well known in the art that information in these communication links may be cryptographically protected by encrypting them with a pseudo-noise (PN) signal which is pseudo-random in nature. For example this may be accomplished by performing an exclusive-or operation of an information signal with a PN signal, prior to transmission. Subsequently, the inverse operation can be performed during the receiving process.
In addition, another encryption technique which is used in the authentication process is described in the United States Digital Cellular (USDC) standard (known as IS-54 and IS-55) and published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006. The USDC encryption technique utilizes a series of specialized messages which must be passed between the subscriber unit and a base site communication unit of the communication system to generate shared secret data (SSD) encryption variables (i.e., encrypting keys known to a subscriber unit and a communication unit which form a communication link) for an authentication (i.e., the SSD.sub.A key) and a voice privacy function (i.e., the SSD.sub.B key).
While the USDC voice privacy encryption process, which utilizes a short, non-changing PN sequence that is repeatedly used to encrypt each successive voice packet, is sufficient for a typically non-redundant voice signal, it is not optimized for use with a highly redundant data stream typical of packetized data communication systems. Packetized data adds an additional problem to the typical encryption process. Packets of data may arrive at different times at a subscriber unit or a base site communication unit because of the unreliability of the physical communication link and because of the algorithms used to compensate for this unreliability. These "packetized" data packets merely need to be reassembled in the same order in which they were created. Therefore, a need exists for an encryption technique which can alleviate the foregoing problems associated with packetized data.